Data loss prevention
DLP. Stop or log sensitive data leaving through the paths you actually instrumented. Policy plus telemetry, not a magic classifier.
Data loss prevention watches endpoints, email, web, and cloud stores for data you labelled or that matches a pattern. It can block, encrypt, or just record. The catalogue of sensitive has to be yours.
It will miss the copy nobody covered, the screenshot, and the mislabelled share. Use it for the noisy, accidental, and lazy. Pair with egress logging and access control for the rest.
See also: Exfiltration, Logging, Privacy Act and OAIC.